This invention relates to transistors and especially to bipolar transistors in which the high-frequency performance is improved by reduction of the base-collector parasitic capacitance.
Bipolar transistors are currently frequency-limited in performance primarily by base resistance, collector capacitance and transit time according to the well-known formula: ##EQU1## where
r is the base resistance
C is the combination of collector-base depletion junction capacitance and collector-base parasistic capacitance
t.sub.e is the emitter junction charging time
t.sub.b is the base transit time
t.sub.s is the collector-depletion-region transit time
t.sub.c is the collector-depletion-region charging tome.
Of these factors, r, C, and t.sub.s are the largest contributors to lower performance. Imposition of the constraints of room-temperature and conventional voltage levels leaves only r and C available for significantly improving the maximum frequency of operation.
The power-added efficiency of the bipolar microwave transistor depends significantly on the emitter injection efficiency (i.e., the ratio of electron current to hole current across the forward-biased emitter-base junction). As the base resistance (r) is lowered to improve maximum frequency of response, emitter injection efficiency becomes proportional to the doping concentration in the emitter region divided by the doping concentration in the base region. This relationship is, however, accurate only for the conventional homojunction.
A method of improving this injection efficiency is to replace the conventional emitter-base homojunction with a heterojunction. In the heterojunction configuration, the emitter material is chosen to have a higher bandgap than does the base material. As such, for a given forward bias, the electron injection into the base is orders of magnitude higher than is hole injection into the emitter.
Another method of obtaining high injection efficiency is to use a tunnel injection. The effect is the same but the injection mechanism differs. Recently a phophorus-and-oxygen-doped polycrystalline silicon emitter was shown to have high injection efficiency. This device was described as a heterojunction injection device.
The polycrystalline SIPOS emitter "heterojunction" device mentioned above may not be a heterojunction at all. A more realistic explanation is that small phosphorus-doped silicon crystallites are surrounded by silicon oxides and phosphorus oxides. These oxides are very thin (e.g., 5 A&lt;t&lt;20 A) and, under forward bias, electrons tunnel through these oxides to the base region. Thus the operation of the device is that of tunnel injection rather than of heterojunction injection. As such, better performance would be obtained if a single crystalline base were used and if the oxide layer could be made very uniform. The problem of interface states would, unfortunately, remain.
To significantly improve the performance (i.e., maximum operating frequency and efficiency or gain), one must reduce base resistance, improve injection efficiency, reduce base-to-collector capacitance, and decrease delay times. In conventional homojunction devices, the first two objectives are not simultaneously achievable. The r-C time constant must be reduced for better microwave performance. In order to avoid impairing injection efficiency when reducing r, a non-homojunction approach must be used. Previous non-homojunction approaches have been effective in simultaneously reducing r while keeping injection efficiency very high, but have suffered by introducing interface states (i.e., electron and/or hole traps) in the metallurgical region between the emitter and base junction. Various schemes have proposed using an AlGaAs emitter on a GaAs base but these approaches (while eliminating interface states) create a problem in making good ohmic contact to the high-bandgap AlGaAs emitter material.
The present invention approaches the problem of improving high-frequency performance from the point of view of decreasing the parasitic capacitance between base and collector of the bipolar transistor.